Regardless of the type of internal combustion engine (for example reciprocating piston engine, pistonless rotary engine or free-piston engine), noises are generated as a result of the successively executed strokes (in particular intake and compression of the fuel-air mixture, combustion and discharge of the combusted fuel-air mixture). On the one hand, the noises propagate through the internal combustion engine in the form of solid-borne sound and are emitted on the outside of the internal combustion engine in the form of airborne sound. On the other hand, the noises propagate in the form of airborne sound together with the combusted fuel-air mixture through an exhaust system that is in fluid communication with the internal combustion engine.
These noises are often regarded as being disadvantageous. On the one hand, there are statutory provisions on protection against noise to be observed by manufacturers of vehicles driven by internal combustion engines. These statutory provisions normally specify a maximum sound pressure for an operation of a vehicle. Manufacturers, on the other hand, try to impart a characteristic noise emission to an internal combustion engine driven vehicles of their production, with the noise emission fitting the image of the respective manufacturer and being popular with customers. Present-day engines with small displacement often cannot naturally generate such intended characteristic noise.
The noises propagating through the internal combustion engine in the form of solid-borne sound can be muffled quite well and are thus usually no problem as far as protection against noise is concerned.
The noises traveling together with the combusted fuel-air mixture in the form of airborne sound through the exhaust system of the internal combustion engine are reduced by exhaust mufflers located ahead of the exhaust system discharge opening (usually a tailpipe) and downstream of catalytic converters, if present. Respective mufflers may for instance work according to the absorption and/or reflection principle. The disadvantage of both operating principles is that they require a comparatively large volume and create a comparatively high resistance to the combusted fuel-air mixture, resulting in a drop of the overall efficiency of the vehicle and in increased fuel consumption.
For quite some time, so-called anti-noise systems have been developed as an alternative or supplement to mufflers, which superimpose electro-acoustically generated anti-noise on airborne noise generated by the internal combustion engine and propagated through the exhaust system. Respective systems are for instance known from the following documents: U.S. Pat. No. 4,177,874; U.S. Pat. No. 5,229,556; U.S. Pat. No. 5,233,137; U.S. Pat. No. 5,343,533; U.S. Pat. No. 5,336,856; U.S. Pat. No. 5,432,857; U.S. Pat. No. 5,600,106; U.S. Pat. No. 5,619,020; EP 0 373 188; EP 0 674 097; EP 0 755 045; EP 0 916 817; EP 1 055 804; EP 1 627 996; DE 197 51 596; DE 10 2006 042 224; DE 10 2008 018 085; and DE 10 2009 031 848.
Respective anti-noise systems typically use a so-called Filtered-X, Least Mean Squares (FxLMS) algorithm trying to bring the airborne noise propagating through the exhaust system down to zero (in the case of noise-cancellation) or to a preset threshold (in the case of influencing noise) by outputting sound using at least one loudspeaker being in fluid communication with the exhaust system. For achieving a completely destructive interference between the sound waves of the airborne sound propagating through the exhaust system and the anti-noise generated by the loudspeaker, the sound waves originating from the loudspeaker have to match the sound waves propagating through the exhaust system in amplitude and frequency with a relative phase shift of 180 degrees. If the anti-noise sound waves generated at the loudspeaker match the sound waves of the airborne noise propagating through the exhaust system in frequency and have a phase shift of 180 degrees relative thereto, but do not match the sound waves in amplitude, only an attenuation of the sound waves of the airborne sound propagating through the exhaust system results. The anti-noise is calculated separately for each frequency band of the airborne noise propagating through the exhaust pipe using the FxLMS-algorithm by determining a proper frequency and phasing of two sine oscillations being shifted with respect to each other by 90 degrees, and by calculating the required amplitudes for these sine oscillations. The objective of anti-noise systems is that the cancellation or influencing of sound at least outside of, but, as the case may be, also inside the exhaust system, is audible and measurable. The term “anti-noise” used in this document serves to distinguish airborne sound generated as a result of the successively executed strokes of the combustion engine and propagated in the exhaust system from airborne sound generated by the anti-noise system and propagated in the exhaust system. In itself anti-noise is just plain airborne sound. It is pointed out that the present invention is not limited to the use of an FxLMS algorithm.
Sound waves also occur in intake systems of internal combustion engines, which may be regarded as annoying. These sound waves are caused by both turbulences in the flow of air and the internal combustion engine itself. The intake system, also called induction tract, includes all combustion air guiding components of an internal combustion engine located ahead of the combustion chamber or combustion space.
An anti-noise system for influencing sound waves propagating through an exhaust system of a vehicle driven by an internal combustion engine is already known from document EP 2 108 791 A1 and is explained below with reference to FIGS. 1 and 2.
The anti-noise system shown in the schematic perspective view of FIG. 1 includes a sound generator 3 in the form of a rigid enclosure comprising an electrodynamic loudspeaker 2 and being connected to an exhaust system 4 by a Y-pipe 1. The Y-pipe 1 comprises a discharge opening 5 at the base of the “Y” for discharging exhaust gases flowing through the exhaust system 4. By having the connection implemented with the Y-pipe, the thermal stress of the loudspeaker 2 disposed within the sound generator 3 that is caused by the exhaust gases flowing through the exhaust system 4 is kept low. This is required because conventional loudspeakers are configured for an operation within a range of up to a maximum of 200° C. only, while the temperature of the exhaust gases flowing through the exhaust system 4 may be up to between 400° C. and 700° C.
FIG. 2 shows a schematic cross section through a sound generator 3 using the example of a voice coil loudspeaker. As can be seen, the loudspeaker 2 comprises a permanent magnet 21 and a funnel-like membrane 22 which are both supported by a loudspeaker basket 23. Hereby, the membrane 22 is connected at its radial outside to the loudspeaker basket 23 by an elastic surround (not shown) and comprises at its radial inside a voice coil (not shown) that moves in a bore in the permanent magnet 21. By applying an alternating current to the voice coil, a Lorentz force is exerted onto the membrane 22 by the voice coil causing the membrane 22 to oscillate. The loudspeaker basket 23 is at its radial outside supported by a bell mouth 42 that is connected to the Y-pipe 1 via a connecting pipe 41. The use of bell mouth 42 is required in this prior art document, since the area of the loudspeaker's 2 membrane 22 is larger than the cross-sectional area of the exhaust system 4 in the sound coupling region. This is necessary in this prior art document to achieve the required sound energy flux.
With the above arrangement, there is a risk of the membrane and/or the voice coil being subject to mechanical damage due to an excessive displacement of the membrane and the voice coil supported by the membrane. This may be caused by external influences like for instance an immersion of the discharge opening (e.g. tailpipe) of the exhaust system in water or a clogging of the exhaust system with, for example, insulating material of a passive muffler. Also, for example, a high ambient air pressure may result in a formation of negative pressure inside the sound generator at the side facing away from the bell mouth altering the oscillation characteristics of the membrane and biasing it towards the permanent magnet. Finally, also a control signal used for operating the loudspeaker may cause an excessive displacement of the membrane, for example, when the control signal is not well enough adapted to the loudspeaker used, or when the control signal stimulates a self-resonance of the loudspeaker.